Early work on polycarbonates was done by Carrothers and Van Nata, III GLYCOL ESTERS OF CARBONIC ACID, Journal of the American Chemical Society, Vol. 52, 314-26 (1930), HIGH POLYMERS Vol. 1, Collected Papers of Wallace H. Carrothers on Polymerization, Interscience Publishers, Inc., 1940, 29-42. It was not until about 1960, however, that polycarbonate resins became available in commercial quantities.
Polycarbonates are members of the larger class of polymers known as polyesters. Commercial polycarbonates are generally polymeric combinations of bisphenols (bi-functional phenols) linked together through carbonate linkages. They are manufactured by ester exchange between a diphenyl carbonate and a dihydroxy aromatic compound, or by phosgenation of a dihydroxy aromatics. The base polymer made from bisphenol A has the following structure: ##STR1##
Industrial polycarbonates are produced in the United States by General Electric Company and sold under the trade name Lexan, by Mobay Chemcial Company and sold under the trade name Merlon, in Germany by Farben Fabriken Bayer AG and sold under the trade name Makrolon, and in Japan by Mitsubishi Edogawa Chemical Company and sold under the trade name Jupilon, and by Idemitsu Kosan Company, Ltd. and Teijin Chemical Company, Ltd. and sold under the trade name Panlite. The common commercially available polycarbonates generally have molecular weights ranging up to about 35,000. Polycarbonate technology generally is quite well developed and reference may be made to texts and to the chemical literature generally for polycarbonate technology details.
Copolymers of polycarbonates and polyalkylene oxides have also been reported, see Eugene P. Goldberg, ELASTOMERIC POLYCARBONATE BLOCK COPOLYMERS, Journal of Polymer Science: Part C, No. 4, pp. 707-730 (1964) and French Patent No. 1,198,715, Eugene Paul Goldberg, publication date Dec. 9, 1959. Goldberg reported the formation of elastomeric polycarbonate block copolymers formed by copolymerization of bisphenol A with poly(oxyethylene), poly(oxyethylene-oxypropylene), poly(oxypropylene), and poly(oxybutylene) glycols. The copolymerization was carried out by the reaction of bisphenol A and the polyalkylene glycol with phosgene in pyridine solution.
Some of the properties of the polycarbonate copolymers are reported by Goldberg, but integral, microporous, high void volume membranes for microfiltration and ultrafiltration, electrophoresis were not previously known and none are suggested by Goldberg.
Polycarbonate membranes, made by a wet process from commercially available polycarbonates, i.e., General Electric Lexan polycarbonates, have been described, Alan Sherman Micheals, South African Patent No. 68/5860, U.S. Pat. No. 3,526,588.
Asymmetric (i.e., skinned) membranes of the copolycarbonates of bisphenol-A and polyethylene oxides prepared by a wet process were reported earlier by R. E. Kesting, the present inventor.
1. R. E. Kesting, paper presented at Cal Tech Symposium on Biomedical Polymers, July 8, 1969;
2. R. Kesting, J. Macromol Sci. (Chem.), A (3), 655, (1970); and
3. R. E. Kesting, Chapter in Biomedical Polymers, ed. by Rembaum and Shen, Marcel Dekker, Inc., New York, 1971 (for dialysis applications).
These membranes, however, were low void volume membranes, i.e. below 50% void volume, or weak, non-integral membranes. A good general reference on membranes is R. E. Kesting, SYNTHETIC POLYMERIC MEMBRANE, McGraw-Hill, 1971, which is incorporated herein by reference.
It has been speculated that polycarbonates are suitable generally as membrane formers. For example, Murata (U.S. Pat. No. 3,450,650) and Seiner (U.S. Pat. No. 3,655,591) speculate that polycarbonates can be made into membranes. Murata, whose basic process is disclosed in U.S. Pat. No. 3,031,328, makes a general speculation concerning many polymers including the polycarbonates. However, I have shown that commercially available polycarbonates are not of sufficiently high MW or solubility to be made into integral high void volume, membranes of great strength and uniformity. Using the procedure referred to by Murata, only non-integral (i.e., lacy) "membranes" with large irregularities, or at best membranes fraught with orange peel imperfections can be produced. Such "membranes" are not of commercial or other practical interest. Seiner's speculation is insufficient to permit evaluation.
As yet, neither skinned or skinned membranes of this type nor any dry process for forming any polycarbonate membranes has been reported, and, until the present invention, unsupported, integral microporous high void volume microfiltration, ultrafiltration and electrophoresis membranes manufactured by a dry (essentially complete evaporation) process and possessing the advantages of the physical and chemical properties of the polycarbonates and polycarbonate copolymers have eluded workers in this field. A principal feature of this advantage is that such integral high void volume membranes having great and unexpected advantages over other membranes can now be produced using the methods, techniques and materials of this invention as set forth hereinafter.
Polycarbonate membranes produced according to this invention are exceptionally tough. The membranes exhibit a heretofore unattainable degree of integrity with high void volumes, i.e., above 50% void, preferably over 65%, ideally 70-85%. The physical properties, such as strength, stability, etc. of the polycarbonates and polycarbonate copolymers combined with the temperature stability and chemical properties of polycarbonates result in membranes produced according to this invention which are far superior in most respects to membranes produced with other materials and using other techniques.
Among the features of the invention which distinguish it from the prior art is the high void volume attainable with the materials and techniques of this invention, coupled with membrane integrity.
The term "void volume" as used herein refers to that portion of the membranes which is occupied by space, i.e. the open or empty portions of the membrane. The term "high void volume" as used herein means membranes having void volumes of greater than 50%, ranging generally in the range of from about 65% to about 85% and preferably in the range of from about 70% to about 85% void volume. A membrane is regarded as an integral membrane, or one having a high integrity, when the membrane is self supporting in one piece with no macroscopically observable discontinuities and having a generally homogeneous structure along any plane taken parallel to the surface of the membrane. (See also under definitions)
Another characteristic which distinguishes some membranes of the present invention from those of the prior art is the absence of a skin. Unskinned membranes are recognizable by their matte finishes on both top and bottom surfaces. Both surfaces are matte because they have a high density of pores in the micrometer (.mu.m) size range. Such porous surfaces do not reflect light as efficiently as dense films or skinned membranes, both of which have a glossy finish.
The unskinned membranes, which are an important facet of the invention, are useful as microfiltration membranes, i.e., in separation processes in which pressure is employed as the driving force such that particulate matter larger than the size of the pores of the membrane cannot penetrate the membrane. Selective separation on the basis of particle size is accomplished using these microfiltration membranes.
The process of this invention is also useful in making high quality, high void volume, microporous integral skinned membranes and, in its broader aspects, this invention contemplates and includes such membranes.
The membranes described herein are useful as microfiltration and ultrafiltration membranes and also as electrophoresis membranes, i.e. membranes used to contain charged (usually proteinaceous) materials in such a manner as to minimize thermal diffusion while the materials move along the membrane as a result of applied electromotive force and the passage of electrical current.
The membranes of this invention consist essentially of polycarbonate resins selected from the group consisting of ##STR2## wherein n is an integer greater than about 180 and less than about 600, x is an integer greater than about 180 and less than 600, y approximates unity where z=80 and approximates 6 where z=13 (for the case where x +z=180), and the sum of x and y is an integer greater than about 180 and less than about 600, z is an integer of from about 13 to about 450, R is a radical selected from the group consisting of EQU H--, (CH.sub.3).sub.2 CH--
and ##STR3## R' is a radical from the group consisting of ##STR4## or substituted derivatives thereof and wherein R" is a radical selected from the group consisting of ##STR5## wherein a and b are integers ranging from about 3 to 10 and c is an integer ranging from about 20 to about 50.
Reference to the compound ##STR6## in this specification and in the claims includes substituted derivatives thereof, such as ##STR7## wherein one or more of the substituents R.sup.A may be hydrogen, substituted or unsubstituted lower (1-9 carbon) alkyl, aryl, lower (1-9 carbon) aralkyl, halogen, nitro, alkoxy or other substituents which do not significantly alter the essential polymer forming and polymer-influencing characteristics of the radical.
The polymers are produced, preferably, by the reaction of bisphenol A or 2,4 tetramethylcyclobutanediol or other appropriate equivalent bisphenols or diols with phosgene in pyridine solution to form homopolymers having molecular weights greater than about 46,000 ([.eta.]=75) or by reacting bisphenol A or 2,4 tetramethylcyclobutanediol and a selected polyalkylene glycol with phosgene in pyridine solution.
Common commercially available polycarbonates, which have molecular weights in the range up to about 35,000 are unsatisfactory and do not form the high void volume microporous integral membranes of this invention (when the dry process is used in their manufacture). Because the dry (essentially complete evaporation) process employs dilute solutions of polymer, polymer MW must be higher than for wet processes which employ concentrated solutions in order to ensure integrity. It is, accordingly, necessary to use polycarbonates and polycarbonate copolymers having molecular weights of at least about 46,000 ([.eta.]=75) and preferably in the range of from about 70,000([.eta.]=100) to 145,000([.eta.]=190), optimum molecular weight being in the range of from about 110,000[.eta.]=150 to about 130,000[.eta.]=175.
The membranes formed according to this invention have many unique and highly advantageous properties, especially as compared with presently commercially available membranes. The membranes of this invention can be autoclaved at substantially higher temperatures than cellulosic membranes and heat sealed. These membranes are highly resistant to hydrolysis, (and can be made more so both by varying R'--R" and by utilizing thiophosgene instead of phosgene) and are very much stronger than known membranes and are non-friable. These membranes are flexible, both wet and dry, do not change dimensions significantly between the wet and dry conditions, and retain their flexibility over a broad range of temperatures. Indeed, the membranes are self supporting, flexible, and retain high strength even at liquid nitrogen temeperatures. This broad range of high flexibility coupled with high dimensional stability, with little change from dry dimensions to wet dimensions, thereby obviates many of the difficult problems which have faced workers using ultra-filtration and electrophoresis membranes in the past.
These and other important features and advantages of the polymers, films, membranes, methods and techniques of this invention will be discussed and will be apparent from the detailed discussion which follows.
Currently available membranes are prepared from a number of different polymers, by far the most important of which are the nitrate and acetate esters of cellulose or blends of cellulose nitrate and cellulose acetate. These membranes are prepared by the phase inversion process, described in detail hereinafter.
The advantage of the cellulose nitrate and cellulose acetate membranes is that they can be prepared in a wide range of pore sizes with a narrow pore size distribution. Their disadvantages include the fact that they are prepared from a natural product starting material, cellulose, which varies rather significantly from batch to batch with the source, climatic conditions, impurities, handling, etc. In addition, the degree of substitution (which can vary from 0 to the unsubstituted cellulose to 3 for the trisubstituted derivative) can likewise vary from batch to batch with observable effects upon processing and end use characteristics. A further disadvantage of the cellulosics for electrophoretic applications is the presence of charged groups such as sulfate or carboxyl. Such groups contribute to electroosmosis which can be a detrimental. Polycarbonates do not have such charged groups.
The polycarbonates, on the other hand, have outstanding physical properties such as tensile and structural strength. Because they are not friable, as are the cellulosics when in an ultrafiltration or electrophoresis membrane form, they are more "forgiving" during handling. Furthermore, their extreme toughness and lack of brittleness makes it possible to prepare useful continuous tapes or rolls of these materials and allows folding and creasing without cracking.
The polycarbonates are more resistant to hydrolysis than the cellulosics and can be made orders of magnitude more resistant to hydrolysis. They are much less biodegradable then the cellulosics and have less tendency to sorb proteins and, therefore, have less tendency to become blocked by proteinaceous slime. The polycarbonates are also heat sealable and autoclavable, retain their flexibility even at cryogenic temperatures and, because of their great strength, do not require reinforcement with fibers, etc., which make fabrications exceedingly difficult and significantly alter characteristics of the end product membranes.